The exemplary embodiment relates to electronically controlled powershift transmissions for large off-road vehicles. More particularly, the exemplary embodiment relates to a system and a method for shifting the transmission of an agricultural or earth moving vehicle by controlling the engagement of a plurality of clutches in accordance with vehicle loading.
In the field of transmission systems, a number of transmission configurations and control schemes have been proposed and are presently in use. Such transmissions typically include a collection of intermeshing gears either fixed to transmission shafts or rotating freely on the shafts. Clutches associated with the freely rotating gears may be selectively engaged to establish a series of speed ratios between an engine output shaft and a transmission output shaft to transmit engine torque at a desired speed to driven wheels of the vehicle. Control systems for commanding engagement of the clutches typically include electronic circuitry that responds to operator controls, such as a shift lever, a direction lever and the like in the vehicle cab. The control system sends electronic signals to hydraulic valves that channel pressurized fluid to the clutches. The control systems thus cause the clutches to engage and disengage in predetermined combinations to accelerate, decelerate and drive the vehicle as desired by the operator. Transmissions and control systems of this type are described in U.S. Pat. No. 4,425,620, issued on Jan. 10, 1984 and assigned to Steiger Tractor, Inc., and U.S. Pat. No. 4,967,385, issued on Oct. 30, 1990 and assigned to J.I. Case Company.
Direct shifting between gears is often provided in transmissions such as those described above. This process, called xe2x80x9cpower shiftingxe2x80x9d, involves disengaging a first set of one or more clutches (the xe2x80x9coff-going clutchesxe2x80x9d) while substantially simultaneously engaging a second set of one or more clutches (the xe2x80x9con-coming clutchesxe2x80x9d). Powershift transmissions are particularly useful for a wide variety of off-road vehicles including, but not limited to, large agricultural vehicles and construction vehicles. Large agricultural vehicles include, but are not limited to, tractors, combines, sprayers and bailers. Representative construction vehicles include, but are not limited to, bulldozers, road graders and earth movers.
These powershift transmissions typically include a number of proportionally-engaged clutches. In general, proportional engagement is accomplished by metering hydraulic fluid to the clutches in response to a shift command. The speed at which the clutch is engaged is controlled by the metered amount of hydraulic fluid entering the clutch. Thus, by carefully controlling fluid pressure entering a clutch, clutch engagement is controlled and smooth transmission operation is achieved. While smooth operation is achievable through careful fluid metering and pressure control, this control is not without complications. For example, these transmissions require valves with orifices for regulating pressure. These valves require complicated calibration routines and are prone to failure.
As mentioned, powershift transmissions including proportional clutches typically provide for multiple forward and reverse gear ratios. Shifting between any of the forward or reverse gear ratios, or between neutral and a forward or reverse gear ratio, typically involves engaging various combinations of the proportionally engaged clutches to achieve the desired forward or reverse gear ratio.
During operation, agricultural and construction vehicles experience a wide range of loading conditions. For example, a tractor may be heavily loaded by a fully-engaged implement, partially loaded by partial implement engagement or rolling implement applications, or lightly loaded during transport operations. In addition to variable loading conditions, these vehicles are operated at a wide range of throttle conditions including part-throttle and full-throttle.
To avoid excess wear to a vehicle, vehicle loading must be determined to properly engage clutches within the powershift transmission. This is because the load on the vehicle influences how quickly the shift should be executed. For example, if the vehicle is lightly loaded, a rapid engagement of the desired proportional clutch will cause the vehicle to xe2x80x9clurchxe2x80x9d significantly as the shift is completed. Lurching stresses both the internal components of the powershift transmission and also the drive line components of the vehicle. Further, lurching produced by rapid engagement can add to operator fatigue as the vehicle is operated over a prolonged period of time.
A simple solution would be to merely engage the clutch slowly. However, where a vehicle is heavily loaded, a slow engagement of the desired clutch will cause almost instant deceleration of the vehicle, thus producing a significant, momentary xe2x80x9cjoltxe2x80x9d as an off-going clutch disengages while an on-coming clutch is slowly brought to complete engagement. This condition, similar to the aforementioned rapid engagement under light loading, excessively stresses both the power transmission and the drive line components of the vehicle. Additionally, the speed of the vehicle and/or engine torque may drop significantly during the time interval between the off-going clutch disengaging and the on-coming clutch fully engaging, thus causing the engine torque to drop below the peak point.
Therefore, it is desirable to control the engagement timing of a clutch as a function of vehicle loading. Accordingly, where the vehicle is operating under a no-load condition, the clutch should preferably be engaged later to produce a xe2x80x9csmoothxe2x80x9d shift, and to prevent lurching. Conversely, where the vehicle is heavily loaded, the clutch should be engaged more quickly than during a no-load condition to avoid sudden deceleration of the vehicle as the shift is executed. Also, clutch engagement should be controlled between the extremes of heavy and light loading.
Significant effort has been expended to resolve the aforementioned powershift transmission problems. The conventional solutions have focussed on controlling the timing of upshift engagement of clutches in power transmissions. While the conventional solutions dramatically decrease wear during upshifts, wear during downshifts remains significant. This excessive wear to both the power transmission and the drive line components of the vehicle has been reduced where upshift control has been replicated to control a downshift of the same gears. For example, the control associated with a shift from fourth gear to fifth gear is replicated to control a shift from fifth gear to fourth gear. While this reduces wear, the wear is still excessive.
For example, one existing solution incorporates a table value used for both upshifting and downshifting. When the shift is commanded, the table is accessed to provide the appropriate clutch engagement timing. The same timing is used for both up-and downshifts between the same gears.
An expanded version of the aforementioned solution provides multiple table values associated with different levels of vehicle loading. Thus, when a shift is commanded, the table is accessed to provide appropriate clutch engagement timing for a specified vehicle load level. However, this is somewhat complicated by the difficulty of adequately ascertaining vehicle loading. While traditional powertrain systems employ a variety of sensors to determine engine and transmission operating conditions, at present it is difficult to directly measure the vehicle loading. Therefore, it is necessary to determine the vehicle loading from known engine operating conditions.
Various methods have been developed to indirectly determine vehicle loading. For example, one method depends upon monitoring a turbocharger employed as part of the vehicle engine. More specifically, the rate of engine exhaust gas flow increases causing the turbocharger to draw in a greater amount of ambient air as the engine rpm increases. The increase in ambient air allows the turbocharger to develop a greater boost pressure in the intake manifold of the engine. Since the boost pressure increases almost as quickly as the engine torque develops, the boost pressure at any given time itself represents a very good approximation of the torque being generated at the same instant by the engine. Thus, vehicle loading at any given time may be approximated by empirical analysis of data received from monitoring turbo boost pressure at a given throttle position, typically full throttle. Using this technique, an accurate engine torque, and hence vehicle loading can be determined when the engine is operating at full throttle.
While this method is useful, it is not applicable where the engine does not include a turbocharger. Further, the method only operates properly when the vehicle is under full throttle conditions. Where full throttle conditions do not exist, this technique does not accurately approximate engine load, resulting in an inconsistent shift as heretofore described.
Thus, a new apparatus and method for controlling shifting in a powershift transmission is needed. In particular, it is desirable to eliminate pressure metered valves. Further, there is a need for an apparatus and method for adequately controlling both upshifts and downshifts of a powershift transmission, with the upshifts and downshifts provided in accordance with the vehicle loading. Thereby, consistent shifts between various gear ratios of the transmission are achieved over various engine load and throttle conditions. Still further, there is a need for an apparatus and method for monitoring engine loading and to thereby obtain an accurate approximation of vehicle loading. Vehicle load should be obtainable at various throttle positions and not depend upon the presence of a turbocharger, the accurate vehicle loading information being useful to control the rate of engagement of a proportional clutch during both upshift and downshift associated with the pertinent clutch.
In accordance with an exemplary embodiment, a method of controlling a plurality of clutches in a powershift transmission is disclosed. The transmission is coupled to an engine of a work vehicle. The method provides for receiving a shift command having a shift direction; selecting clutch timing based on the shift direction of the received shift command; engaging an on-coming clutch; and disengaging an off-going clutch before, after, or during engaging the on-coming clutch, wherein the time delay, if any, between engaging the on-coming clutch and disengaging the off-going clutch is based on the selected clutch timing.
In accordance with another exemplary embodiment, an apparatus for controlling operation of a powershift transmission is disclosed. The transmission is coupled to an engine of an off-road vehicle. The apparatus includes a sensor for sensing vehicle load. The invention further includes a plurality of on/off valves for controlling fluid flow to a plurality of clutches in the transmission. A table of clutch parameters corresponding to vehicle load and clutch characteristics is utilized to control sequencing of valve energization. Sequencing of valve energization provides clutch sequencing that provides for smooth shifting.
In accordance with yet another exemplary embodiment, an electronic transmission control system for controlling clutch sequencing in a powershift transmission is disclosed. The transmission is coupled to an engine of a work vehicle. The control system includes a sensor configured to sense vehicle load and to provide a load signal. The control system further includes a plurality of tables having clutch timing parameters and a controller. The controller is configured to receive the load signal and to select one of the plurality of tables based on the load signal. Each table corresponds to a first range of vehicle loads during an upshift and a second range of vehicle loads during a downshift, the first and second ranges being different. The controller controls the sequencing of the clutches based on a clutch timing parameter from the selected table.
In accordance with yet another exemplary embodiment, an apparatus for controlling a plurality of clutches in a powershift transmission is disclosed. The transmission is coupled to an engine of an off-road vehicle. The apparatus includes a means for receiving a shift command. In addition, the method includes a means for selecting clutch timing that is dependent upon shift direction of the received shift command. Further, a means for engaging and disengaging clutches is included. The selected shift timing is used in conjunction with the means for enganging and disengaging to provide smooth shifting.